Mercury boiler



June 26, 1934. A. J. NERAD MRGURY BUILER mea Aug.. 17,4931

ratented June 26, 1934 UNITED STATES MERCURY BOILEB Anthony J. Nerad, Schenectady, N. Y., assigner to General Electric Company, a corporation of New York Application August 17, 1931, Serial N0. 557,604

10 Claims.

In connection with the vaporization of mercury in mercury boilers, one limitation has been that imposed by the limited ability of the mercury to absorb heat from the heated surfaces, for example, the boiler tubes. The rapidity with which the mercury absorbs heat from the heated surfaces determines the capacity or rating of a given size of boiler and it is clear that if the ability of the mercury to absorb heat from the heated surfaces is increased, the capacity of the boiler can be correspondingly increased. y

I have. discovered means whereby the heat transfer of a mercury boiler can be greatly increased, in fact, increased to an extent such that this is no longer the limiting factor in the capacity of the boiler, the limiting factor then becoming, rather, the ability of the mercury boiler to handle the volume of mercury vapor generated.

I have discovered that if in a mercury boiler the surfaces from which the mercury liquid absorbs heat, for example the boiler tube surfaces, are formed of iron or steel and these surfaces are made chemically clean so that they are substantially free from all foreign matter such as oxide scale, the heat transfer capacity of the surface is greatly increased, being increased 100% or more in tubes large enough to handle the increased capacity. I have found that mercury liquid wets a chemically clean iron or steel4 surface in the absence of air at the temperatures and under the conditions at which mercury boilers operate, and it is because of this that the heat transfer capacity is increased. In wetting the iron or steel surfaces, the mercury forms a thin adhering film of mercury on the metal surface whichiilm is in intimate contact with the metal surface. The transfer of heat from the iron or steel to the mercury is effected through the intimate contact of this nlm with the iron and the mercury.

The heat transfer surfaces may be cleaned in any suitable manner. For example, they may be cleaned by pickling, by exposure to hydrogen while hot, or by other process, or they may be cleaned by mechanical means.

Cleaning by exposure to hydrogen for a considerable period of time while hot is particularly effective and advantageous since it decarbonizes the steel for an appreciable depth below the surface and leaves a surface of relatively pure iron which is quickly wet when the boiler is putinto operation. Exposure to hydrogen for a period of a few hours at a temperature of approximately 950 degrees C. is satisfactory for effecting the cleaning operation. I consider the cleaning of steel by hydrogen as being one important feature of my invention in that by cleaning by hydrogen I obtain operating results superior to those obtained when the steel is cleaned by other means.

If any considerable time must elapse between the cleaning of the surfaces and the operation of the boiler I prefer to protect the surfaces from oxidation by smearing or covering them with oil or other substance that will be removed easily 55 when I wish to operate the boiler.

In addtion to the foregoing, I have discovered that the surfaces may be cleaned by adding to the mercury an alkali metal such as sodium or potassium, thereby providing an amalgam. I have found by extensive tests that the presence of a metal such as sodium or potassium in the mercury effects a cleaning of the surfaces so that the mercury will wet the surfaces. A relatively small quantity of the sodium or potassium is required. For example, in the case of sodium I have found one fourth of one percent of sodium metal by weight to be suicient. The sodium may be added in the form of the metal but this is not necessary as it may be added in the form of sodium hydroxide. An aqueous solution of sodium hydroxide may be used if desired. After the application of heat, sodium amalgam is formed, while the water vapor and hydrogen are driven off and may be removed by means of an air pump. After the surfaces are cleaned mercury may be substituted for the amalgam. However, I have found it advantageous to operate the boiler with the amalgam, as the presence of the amalgam appears to improve the heat transfer, in addition to the improvement effected due to initial removal of the oxide or other coating. The presence of an alkali metal in a mercury boiler causes a continuous cleaning of the boiler walls and thereby forms in substance a means for maintaining the surfaces of the boiler in contact with the mercury clean during operation of the boiler. In addition. the adding of an alkali metal to mercury has another advantageous effect, namely, the diminution of the surface tension of mercury liquid, to which I ascribe at least partly the increased heat transfer from the boiler walls to the mercury. To make this clear, attention is directed to the following physical facts: The heat transfer between m5 a solid and a liquid, in the present case between the boiler wall and the mercury, depends upon the adhesion between the solid and the liquid. This adhesion in turn is a function of the surface tension of the solid and of the liquid. 'I'he 110 ses greater the surface tension, the smaller the adhesion. Hence, a reduction in surface tension of one of the substances contacting each other, especially a reduction of the surface tension of the mercury, increases the adhesion between the mercury and the steel, resulting ina better heat transfer from the steel to the mercury. The adding of an alkali metal such as sodium to mercury causes a diminution of the surface tension of the mercury and consequently an increased heat transfer from steel or iron to mercury.

The addition to mercury of other metals which have a greater aflinity for oxygen at operating temperatures than iron is helpful in reducing and removing the oxide scale from the heat transfer surfaces. I have had especially good results with a boiler coated at the beginning with zinc or tinr applied so as to eliminate the oxide coating. The

zinc or tin of course dissolves off quickly when the tubes containing mercury are warmed up.'

and I ascribe the remarkably successful performance ofV this procedure to the cleaning of the steel preparatory to deposition of the zinc or tin, or in the course of such deposition, and maintaining the surface free from oxide until the zinc is dissolved off by the mercury.

In interpreting the results of my tests I distinguish between the thick coating of oxide and other impurities, that is found on commercial tubes as delivered, and a thin nlm'of oxide that begins to form almost instantly `on clean steel surfaces when they are exposed to the air even at room temperature. The' rapid formation of a thin nlm of oxide at room temperature can be observed by breaking a rod in the presence of air and dipping it into mercury liquid. Although the breaking and dlppina may be performed rapidly. the broken ends of the rod will not be wetted because a thin nlm of oxide forms almost instantly. However, the broken ends of the rod are wetted by the mercury if the break is performed underneath mercury, that is, in the absence of air. vThis thin film of oxide affects adversely the performance of a well cleaned boiler at the outset but after a period of operation (one or more days) the performance improves appreciably. Such improvement is ascribed to disappearance of the oxide film. The addition to the mercury of metals having greater amnity 4for oxygen than iron under operating conditions helps in removal of the oxide film, and in maintaining the surface of the steel free from oxide nlm during long continued operation. In

addition, the adding of such metals to mercury.

liquid, as pointed out above, has a direct effect on the mercury in that it reduces its surface tension, resulting in increased adhesion between the mercury and the steel or. iron walls. of the boiler andaccordingly a better heat transfer from the steel or iron walls to the mercury.

While this oxide nlm may be removed from the beginning and kept from forming again'by galvanizing or tinning the surface of the steel, nevertheless such galvanizing or tinning when it has been dissolved otf by the mercury constitutes a considerable impurity in the mercury. Satisfactory wetting may be secured with a much smaller amount of the added metal if the tubes are well cleaned and the metal having a greater ailinity for oxygen at operating temperatures than mercury is added to the mercury in place of galvanizing or tinning.

Heretofore in the design and construction of mercury boilers there has been met with always the limitation that the mercury did not wet the heat absorbing surfaces and this has been a serious factor in limiting the capacity of a boiler. Also it has been a factor tending to give more or less uncertainty of operation due to the possibility of overheating, a thing which may result in tube failure. By my invention, these diiliculties are removed. At the same time, with the same size of boiler I can double or more than double its capacity, or I can build a boiler of a certain capacity much smaller than heretofore, thus making a substantial saving in cost and in quantity of mercury required. This latter is an especially important consideration due to the relatively high cost of mercury.

In the drawing, the figure is a sectional view of a portion of a mercury boiler embodying my invention. This drawing is of a more or less diagrammatic character. A mercury boiler of the character illustrated is disclosed more fully in the application of Coulson'and Emmet Serial No. 308,486, filed September I7, 1928, and assigned to the same assignee as the present invention.

Referring to the drawing, 1 indicates a boiler drum from which depend a plurality of boiler tubes, only one being shown in the drawing. Each boiler tube comprises an outer tube 2 and a core 3 located in spaced relation to the outer tube to provide an annular passage 4. Core 3 comprises two tubes 5 and 6 joined at their ends 105 to provide a dead air space between them. The core dennes a vertical passage 7 for the down i'iow of mercury. The upper end of core 3 is connected to a baille plate 8 suitably supported in spaced relation to the adjacent wall of the boiler 110 drum. In operation, liquid mercury is fed to core tube 5 from above plate 8 and ows down passage 'I and thence up annular passage 4. Core passage "l thus serves to feed liquid mercury to annular passage 4. The heat is applied to 115 the outer surfaces of tubes 2 and the mercury in flowing up annular passage 4 absorbs heat and is vaporized. This is the known operation of a mercury boiler of this type.

According to my invention, the inner surface m of tube 2that is the surface with which the mercury contacts and from which it absorbs heat, and which may be termed the. heat transfer surface, comprises iron or steel which is chemically clean, it having been cleaned after the manner hereinbefore described. Preferably the tube as a whole is formed of iron or steel although it is essential only that the inner surface or heat transfer surface comprise this material. As already stated, I have found that when the heat transfer surface is chemically clean or free from oxide scale, mercury wets the surface at the temperatures and imder the operating conditions met with during the normal operation of the boiler, with the result that the heat transfer 135 capacity of the surface isvincreased greatly.

It is .desirable that there be as little restriction of circulation as possible of the mercury as it passes up annular passage 4. Since a wetted surface restricts the circulation, the surfaces of no the core are preferably of a material which is not wetted by mercury.

According to another feature of my invention, I add an alkali metal such as sodium or potassium or some other metal having a greater afiinity for oxygen than iron at operating temperature to the mercury, and cause an increased adhesion between the mercury and the steel or iron walls of the boiler b'y reducing the surface tension of the mercury. In the drawing 10 indi- Leagues cates the operating substance of the boiler which may be mercury to which has been added such another metal.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A mercury boiler having as operating sub-'-` stance mercury to which has been added a substance which chemically releases the oxide scale whereby at the temperatures and under the operating conditions met with during the normal operation of the boiler, the mercury wets the heat transfer surfaces.

2. A mercury boiler having as the operating substance mercury to which has been added an alkali metal, whereby at the temperatures and under the operating conditions met with during the normal operation of the boiler, the mercury wets the heat transfer surfaces.

3. A mercury boiler having as the operating substance mercury to which has been added sodium, whereby at the temperatures and under the operating conditions met with during the normal operation of the boiler, the mercury wets the heat transfer surfaces.

4. A mercury boiler having the heat transfer surfaces with which the mercury engages formed of iron or steel, said boiler having as the operating substance mercury to which has been added an alkali metal, whereby at the temperatures and under the operating conditions met with during the normal operation of the boiler, the

A mercury wets the heat transfer surfaces.

5. A mercury boiler having the heat transfer surfaces with which the mercury engages formed of iron or steel, said boiler having as the operating substance mercury to which has been added sodium, whereby at the temperatures and under the operating conditions met with during the normal operation of the boiler, the mercury wets the heat transfer surfaces.

6. The method of cleaning and then maintaining clean during operation the heat transfer surfaces in a mercury boiler which 'comprises cleaning the surfaces by exposure to hydrogen while hot, placing mercury in the boiler, and adding to the mercury in the boiler a substance having a greater amnity for oxygen than has iron at the temperature of operation of a mercury boiler, whereby at the temperatures and under the operating conditions met with during the normal operation of the boiler, the mercury wets the heat transfer surfaces.

7. A mercury boiler comprising heat transfer surfacesV of iron or steel having as its operating substance mercury to which has been added a substance having a greater amnity for engen at the operating temperature than has iron, whereby at the temperatures and under the operating conditions met with during the normal operation of the boiler, the mercury wets the heat transfer surfaces.

8. A mercury boiler having as its operating substance mercury to which has been added a substance which removes the oxide film from the vsurfaces of the boiler that are in contact with the operating substance, whereby at the temperatures and under the operating conditions met with during the normal operation of the boiler, the mercury wets the heat transfer surfaces.

9. A mercury boiler having heat transfer surfaces w'ith which the mercury engages formed of iron or steel which has been cleaned of scale and having as the working substance mercury containing a substance having a greater aillnity for oxygen at operating temperature than has iron, whereby at the temperatures and under the operating conditions met with during the normal operation of the boiler, the mercury wets the heat transfer surfaces.

10. A mercury boiler having heat transfer members with which the mercury engages, the mercury engaging surfaces of said members under operating conditions met with during normal operation of the boiler being formed of chemically clean iron or steel, said mercury cooperating with said surfaces at the temperatures and under the operating conditions met 'with 115 during the normal operation of the boiler to form upon said heat transfer surfaces an adhering film of mercury, whereby transfer of heat from the iron or steel to the mercury is effected through the intimate contact of said lm with the iron and the mercury.

ANTHONY J. NERAD. 

